1. Field of the Invention
The present invention relates to a pump casing made of sheet metal, and more particularly to a pump casing which is made of sheet metal such as stainless steel plate and formed by press working.
2. Description of the Related Art
It has been customary to press sheet steel such as stainless steel into a pump casing according to a deep drawing process, and then weld or otherwise fasten suction or discharge flanges to the pressed pump casing. The pump casing thus fabricated is liable to be deformed due to the internal pressure developed in the pump casing itself and also due to external forces applied to the pump casing from a piping connected thereto. Any deformation that the pump casing suffers should be reduced to such a level that the pump casing will never be in contact with the impeller housed in the pump casing. Therefore, the pressed pump casing must meet mechanical strength requirements for bearing loads including external forces applied from the connected piping and an internal pressure developed in the pump casing, and also rigidity requirements for keeping a desired clearance between a liner ring and the impeller.
Inasmuch as the pressed pump casing is a relatively flexible structure with respect to applied loads, however, it is impossible for the pressed pump casing itself to satisfy both the mechanical strength requirements and the rigidity requirements. To achieve a desired level of rigidity, conventional pressed pump casings have employed a steel sheet whose thickness is greater than the thickness required to withstand an internal pressure developed therein, or a reinforcing member of complex shape disposed between the suction flange and the pump casing for reinforcing them.
One known pressed pump casing with a reinforcing member is disclosed in German DE-OS 3517828. As shown in FIG. 18 of the accompanying drawings, the disclosed arrangement includes a reinforcing member 64 of complex shape, called a box guarder, which is positioned between a pump casing 61 and a suction flange 62 in surrounding relationship to a suction nozzle 63.
The increased thickness of the sheet steel or the reinforcing member disposed between the pump casing and the suction flange is effective to prevent either the pump casing housing the liner ring from being deformed or a liner section of the pump casing from being deformed, so that a desired clearance is maintained between the liner ring or the liner section of the pump casing and the impeller.
As described above, in order to prevent the pump casing or the liner portion thereof from being deformed, the rigidity of the pump casing has been increased by employing either a steel sheet whose thickness is greater than the thickness required to withstand an internal pressure developed in the pump casing or a reinforcing member of complex shape disposed between the suction flange and the pump casing for reinforcing them. The steel sheet of increased thickness is, however, difficult to press to shape, and the reinforcing member of complex shape results in an increase in the cost of manufacture of the pump casing.
On the other hand, there have been known full-circumferential flow pumps which have an annular fluid passage between a pump casing and a motor accommodated in the pump casing. In general, the full-circumferential flow pump has a pump casing comprising a suction-side casing, a discharge-side casing, and an outer cylinder extending between the suction-side casing and the discharge-side casing. The suction-side casing and the discharge-side casing are generally highly rigid as they are each made of a casting such as an iron casting.
The full-circumferential pump also includes a canned (sealed) motor fixedly supported in the rigid suction- and discharge-side casings.
The canned motor can be fixedly supported in the suction- and discharge-side casings because they are in the form of rigid castings. If these casings comprise sheets pressed to shape, on the other hand, the canned motor cannot fixedly be supported in the casings because they are not sufficiently rigid.